Promiscuity in the pleistocene

Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology and his colleagues from various institutions are making stunning advances in the analysis of ancient DNA. The complete genome of a Neanderthal has now been reconstructed with a remarkable level of detail.

Editors Summary: Recent excavations in the Denisova Cave in the Altai Mountains of southern Siberia have yielded a wealth of hominin fossils from a site that has been occupied for perhaps 250,000 years or more. Now a high-quality genome sequence has been determined from a circa 50,000-year-old toe bone — a proximal toe phalanx — excavated from the east gallery of Denisova Cave in 2010. The sequence is that of a Neanderthal woman whose parents were closely related — perhaps half-siblings or uncle and niece. Such inbreeding was also common among her recent ancestors. Comparisons with other archaic and present-day human genomes reveal several gene-flow events among Neanderthals, the closely related Denisovans and early modern humans, possibly including gene flow into Denisovans from an unknown archaic group. The high-quality Neanderthal genome also helps to establish a definitive list of substitutions that became fixed in modern humans after their separation from the ancestors of Neanderthals and Denisovans.

But what is also becoming clear is that there were more “species” of homo erectus who existed in parallel than has generally been assumed and also that sexual encounters and interbreeding between these cousin-species has been a regular occurrence over some 250,000 years. And so there have been times when Anatomically Modern Humans (AMH), Neanderthals, Denisovans and the “unknown” hominim have co-existed.

The AMH-Neanderthals split is thought to have occurred about 400,000 years ago. The split with the unknown hominims then must have been around 500,000 years ago. The Denisovans split off from the Neanderthals perhaps about 300,000 years ago. The Out-of-Africa split among AMH was around 100,000 years ago.

A Neanderthal in our time. (copyright Nenderthal Museum / H Neumann)

These ancient splits were all probably in and around Africa even if the Out-of-Africa event for AMH is now more likely to have been many such events and an expansion out of Africarabia. But it also means that a wave of “unknown hominims” split off from the mainline after a mainline group had left Africa and then spread out to unknown areas. Neanderthals probably split from their parent line of descent also outside of Africa but there must first have come a migration from Africa. The Neanderthal-Denisovan split must also have taken place outside of Africa. Clearly there have been many migrations Out of Africa over the last 400,000 years.

Carl Zimmer writes, “Evolution is a mixture of flow–the cascade of genes from parents to offspring, and the criss-cross movement between populations and species”.

But this generates a myriad of new questions. How large were the troops or tribes or clans of these ancient hunter-gatherers that they could sustain such large migrations? For how long and in what proximity did these different species of man co-exist. These troops – it is thought – probably did not number more than about 50. Some critical population would have been required for sufficient interactions to have taken place between the species. At any one time the total human population – of all species – may have approached 10 million (for if this was – say – just 1 million the opportunities to interbreed would not have been many). They all had fire, but did they all have some form of speech?

But most fascinating of all is how the interbreeding took place. Was it just a product of normal rape and pillage? Did violent clashes lead to the victors impregnating the vanquished, or were there other scenarios for individuals to mate? At some point there must have been children who were 50% Neanderthal and 50% AMH. And some who were 50% Neanderthal and 50% Denisovan. How did they survive? What kind of society existed in these ancient times that would permit such offspring not only to survive but also to mate and produce offspring in their turn? But however it happened, our ancestors in the pleistocene were a promiscuous lot.

Could it be that it was only among AMH – after they had been vanquished and raped and pillaged by Neanderthal raids – that such mixed children were allowed not only to survive but also to thrive?

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This entry was posted on January 2, 2014 at 7:42 pm and is filed under Anthropology, Evolution, Humans. You can follow any responses to this entry through the RSS 2.0 feed.
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